Presently, grinding operations are controlled primarily by establishing timing intervals for each application of an abrasive to an object to be ground and by human observation of the rate at which the abrasives wear away the object being ground. In another control procedure, a grinding operator attempts to set mechanical stops at a level corresponding with the desired grinding depth for each grinding operation.
One such object to be ground is a coupon for a printed circuit board. Printed circuit boards provide the mounting surface and electrical interconnection system for components such as diodes, resistors and capacitors. While originally the circuit boards were only printed on a single side, most circuit boards today are double-sided or multilayered. Multilayer printed circuit boards, that is, circuit boards having circuits that are printed on several layers of the boards, require uniform, dependable electrical connections between the layers. These critical connections are typically provided by plated-through holes prepared by drilling holes through the stacked board layers. The drilled hole are plated with copper and then solder to establish the electrical interconnections. Each and every plated-through hole must maintain a predetermined, uniform thickness: pores, cracks, nodules and other faults in plating render the entire circuit board defective.
A number of printed circuit boards are typically defined on a single panel of material. Fortunately for inspection purposes, plating faults typically appear in most of the plated-through holes of a printed circuit board if any faults are present at all. Therefore, one or more coupons are defined on the panel for each circuit board to be printed. Each coupon has one or more test holes which are plated simultaneously with the plated-through holes. The coupons are labelled with a specific identification code for the individual board and are detached for later examination.
The test holes in the coupon are typically aligned with their centerlines in a plane perpendicular to the direction of grinding. They are examined after plating by accurately grinding one edge of the coupon to expose the test holes in cross-section. However, the greater the distance of the cross-section from the centerline, the greater is the error in measurement of actual plating thickness.
Unfortunately for inspection purposes, grinding is presently a tedious, time-consuming process since great accuracy is demanded. At least three or four steps are involved which are accomplished manually or semi-automatically by human supervision of a grinding machine. Typically, one or more coupons are mounted in a holder with one edge protruding. The holder is cast in a mold after the coupons are carefully arranged in the mold. The coupons are aligned in the mold using alignment pins which pass through tooling holes in the coupon. Potting material is then poured into the mold which hardens to form the holder. Grinding machines accept one or more holders in a disk which is equipped with a number of adjustable mechanical stops including hardened material such as diamond.
The protruding edge of each coupon is ground for several minutes against coarse grit rotating at several hundred rpm. The coarse grit is replaced with medium grit, which is rotated against the coupons for an additional one to two minutes. When a semi-automated machine is used, the operator removes the coupons from the medium grit after all diamond stops contact the grit; the operator then resets the diamond stops. Fine grit is then applied against the coupons for thirty to fifty seconds or until all the diamond stops again contact the abrasive. For the grinding machine, the diamond stops are reset flush with the holder. Finally, the coupons with their test holes exposed in cross-section are treated in one or more polishing steps.
The sheer cost and labor of grinding one or more coupons per circuit board present serious problems in view of the ever-increasing millions of printed circuit boards that are produced annually. Presently, most coupons are ground manually at the cost of $15-20 per coupon. Manual grinding requires constant operator attention and frequent inspection using a microscope. Each visual inspection interrupts the grinding operation. The dependability of the operator varies greatly: overgrinding and undergrinding occur frequently. Since the coupons are exposed destructively, a mistake in overgrinding is irreparable and results in the complete invalidation of the matching printed wire board unless a second coupon is available for complete regrinding. Undergrinding, when detected, is cured by returning the coupon to the operator, who must remount it and commence additional, unscheduled grinding.
Semi-automated or machine-assisted grinding also requires close operator attention. The machines are more dependable than grinding by hand but are not reliable for exposing small test holes due to cumulative sources of error. The tooling holes in one of the coupons can be misplaced relative to the plated-through holes to be examined which results in under- or overgrinding of the coupon. Further, the holder can be initially misaligned in the disk relative to the setting of the mechanical stops, and the stops themselves can wear over time. Also, several holders can be misplaced in a disk relative to each other.
One apparatus attempts to automate grinding by interconnecting two terminal strips with a shorting conductor that is deposited on a substrate to be ground. The grinding operation is controlled by the amount of current flow through the shorting conductor. This technique is relatively inaccurate, however, and relies on final finishing by hand.
Further, the terminal strips and shorting conductor are all deposited on the same face of the substrate. To expose an element passing through the substrate, such as a test hole to be exposed, controlled grinding along the thickness of the substrate is desired. In other words, control is desired along a grinding surface transverse to one or more faces of the substrate.